Extractive distillation with



Nov. 12, 1946. v. scARTH EXTRHCTIVE DISTILLATION WITH FURFURAL OriginalFiled Dec. 21, 1944 BBGHOSQV 2A dELLS INVENTOR V, SCARTH ATTORNEYS zmZdFDm IZ asoma Nav. 12,1946

22,811 sxmc'rrvs ms'rnLA'noN wrm mmm f virgu aww, Barnesville, ons.,mim: u Phillips Petroleum Company, a corporation o! Delaware No.2,404,253, dated July 16, 1946, Serial No. 569,243, December 21, 1944.Application for reissue August 9, 1946, Serial No. 689,319

(ci. 19o-13) 6 Claims. l

This invention relates -to ,extractive distillation,

. more particularly to extractive distillation of hydrocarbon streamscontaining unsaturated hydrocarbons using furfural as the solvent, andstill more particularly to extractive distillation for the recovery ofaliphatic monoolefins and/or aliphatic conjugated diolens especiallynormal butene and/or butadiene from hydrocarbon streams containing samewith iurfural as the solvent.

It has become customary to recover unsaturated aliphatic hydrocarbonsespecially butene and/or butadiene from hydrocarbon streams containingthem by extractively distilling the Stream in a fractional distillationcolumn or absorber into the top of which there is introducedcontinuously a. stream of furiural, usually containing a few per cent ofdissolved water, in amount suiilcient to dissolve the unsaturatedhydrocarbon. The top ofthe column is reiluxed with aI portion oicondensed overhead vapors in the usual way and the bottom of the columnis provided with the conventional reboiler for the purpose of boilingthe rich furfural as it attains the bottom so as to free it fromundesired parailin and/or olefin. The rich furfural is withdrawn and fedto a stripping column where the dissolved unsaturated hydrocarbon isremoved. 'Ihe stripper is provided with reboiler and is refluxed with aportion of the condensed'overhead. The purpose of refluxing theabsorberand stripper is to' prevent carryover of furfural into the overheadvapors. 'I'he stripped furfural is then returned from the bottom of thestripper to the absorber.

During extrae-tive distillation of unsaturated hydrocarbons withi'urfural the furfural becomes contaminated with acids,l polymer andother impur-'ities which are very deleterious because they are highlycorrosive to the metal equipment, The resulting corrosion of expensiveequipment including fractional distillation columns and accessories suchas piping, reboilers, condensers, accumulators, pumps and so forthpresents a serious problem for which various remedies have been proposednone of which is entirely satisfactory, so far as I am aware. The causeof the formation of corrosive impurities in the Iurfural is not knowneven though the problem has been extensively investigated. Factors whichmay conv tribute to the formation include high temperature, presence ofwater and presence of unsaturated hydrocarbons. It is believed by someinvestigators that the furiural'selectively absorbs traces of corrosivematerial from the hydrocarbon feed. or of material from the feed whichbe- 2 It is not desired to be limited to any theory as to the cause ofthe furfural becoming corrosive.

It is conventional practice to purify the furfural continuously byre-running a side stream of the furi'ural leaving theV stripper andrecycled to the absorber. Ihis is carried out in accordance with U. S.Patents to K. H. Hachmuth 2,372,668 and 2,350,609, and Buellet al.2,350,584. However, there is considerable additional loss of furfuralduring the re-running operation. A large portion of this loss is due topolymerization of the furiural in the re-run unit. At present thefurfural is re-run for the purpose of removing other corrosive materialas well as the polymer. It a feasible method of preventing theaccumulation or corrosive material in the i'urfural in the main system(i. e., in the absorbing and stripping operations) were available, theamount of furfural passed to the re-run unit could be materiallydecreased with many advantages principal of which is the reduction ofpolymerization of furfural in the re-running operation. Many otheradvantages of such a method will appear hereinafter.

The principal object of the present invention is to provide an improvedmethod of conducting extractiva distillation of unsaturated hydrocarbonswith `furiural. Another object is to piovide aiea'sible method ofpreventing the accumulation of corrosive material in the furfural in anextractive distillation system wherein unsaturated hydrocarbonsespecially aliphatic monoolefins and dioleflns are separated fromhydro'- carbon streams containing same. Another object is to providesuch an extractive distillation system applied to the recovery ofnormalbutene and/or butadiene by means of furfural. Another object is toprovide a method which greatly reduces the amountof furfural which hasto be passed to the re-run unit. Another object is to prevent corrosionof the' main system (i. e., the extractive distillation system proper asdistinguished from the re-run'unit). Another object is to reduce theloss oi' furfural in the extractive distillation system and in there-running operation. Another object is to prevent the formation o!corrosive impurities in the furfural at the point where thoseimpur-ities are most deleterious. Another object is to accomplish theforegoing objects in a manner which does not cause 'foaming of thefurfural. Numerous other objects will hereinafter appear.

The accompanying drawing portrays diagrammatically one arrangement ofequipment for carcomescorrosive upon reacting with-the furfural. ryingoutthepresent invention.

I have discovered that in the extractive distillation of hydrocarbonstreams with furfural. as the solvent to recover -unsaturatedhydrocarbons therefrom the furfural may be prevented from becomingcorrosive and from corroding the equipment by intimately contacting thefurfural in the extractive distillation system with lime. The term limeas used herein denotes calcium hydroxide. I have determined thatlimestone will not operate in the process of the present invention.

The contacting of the furfural with the lime is commonly accomplished byintroducing the lime in finely divided form into the furfural in anysuitable manner. The lime may be added in the dry form. as a dispersionor slurry in furfural, as a dispersion or slurry in water or by othermethods. Probably the most convenient method is to add it in the form ofa suspension or dispersion in a portion or all of the makeup water whichis commonly added continuously to the furfural fed to the absorber orabsorbers in order to impart the desired content of dissolved water tothe furfural. It will be understood that addition of make-up water isfor the purpose of I replacing water volatilized from the furfural inthe absorbers and strippers. The use of furfural containing dissolvedwater as a solvent in the extractive distillation of normal buteneand/or butadiene is disclosed and claimed in the copending applicationof K. H. Hachmuth, Ser. No. 438,844, led April 13, 1942.

The present invention is applied to the extractive distillation ofunsaturated hydrocarbons with furfural wherever the furfural becomescorrosive due to conditions prevailing in the extractive distillationsystem. The factors which cause the furfural to become corrosive are butlittle understood but it is believed that they include the hightemperatures especially in the reboillng sections of the absorber andstripper, the presence of water and unsaturated hydrocarbons especiallyoleflns and diolens, the presence of air or oxygen. The development ofcorrosivity in the ,furfural is accompanied by the formation of polymerwhich represents a loss of furfural and causes poor heat transfer andother troubles. The present invention is especially applicable to theextractive distillation of butadiene and/or normal butene with furfural,this being the principal if not the o ly commercial application ofextractive distill ion with furfural as a means of recoveringunsaturated hydrocarbons. However, it may also be applied where furfuralis used in the recovery by extractive distillation of other aliphatickoleflns or aliphatic conjugated diolepm from hydrocarbon streamscontaining the same.

The conversion of normal butane to butadiene is now commonly effected bya series of steps including (I) catalytic dehydrogenation of normalbutane to normal butene, (II) recovery of normal butene and recyclenormal butane by rst fractionally distilling the C4 content of theellluent from step I in a butene-1 column wherein butene- 1 is recoveredoverhead followed by extractive distillation of the resulting bottomsfraction of nbutane and butene-2 with furfural to separate the butene-2from the normal butane which is recycled to step I, (III) passing thecombined butene-l and butene-2 recovered in step II to a secondcatalytic stage and therein dehydrogenating same to butadiene, andfinally (IV) recovering butadiene from the C4 content of the eilluent ofstep III by a sequence of steps ,including depropanizing, debutanizing,extractive distillation of the debutanizer overhead with furfuralwhereby the butadiene and butene-2 content aredissolved in the furfuraland subsequently recovered therefrom by the stripping operation, andfinally fractionally distilling the butadiene-butene-Z mixture toseparate pure butadiene from butene- 2. This process is fully describedin the copending application of K. H. Hachmuth, Ser. No. 481,- 305,filed March 31, 1943, which has now matured into Patent No. 2,386,310,dated Oct. 9, 1945. The present invention is applicable to the furfuralextractive distillation system in step II or in step IV or in both. Orit may be applied in those butadiene plants where normal butylene feedis catalytically dehydrogenated to butadiene and butadiene is recoveredfrom the effluent by extractive distillation with furfural.

In such an extractive distillation process the furfural enriched inunsaturated hydrocarbon is withdrawn from the bottom of the absorber andpassed to the stripper where the unsaturated hydrocarbon is strippedtherefrom. The stripped furfural is then cooled and after additionofmakeup water is recycled to the absorber. Usually a surge tank isprovided in the recycle line. A sidestream of the stripped furfural iscontinuously fed to a furfural re-run unit wherein it is steamdistilled, e. g. in the manner of Buell et al. 2,350,- 584, to give apurified material consisting essentially of furfural and dissolved waterand free from polymer and other material relatively nonvolatile withsteam. The re-run furfural is then merged with the main recycle streamfor passage to furfural storage and ultimately to the absorber. Thevolume of furfural passed to the re-run unit is preferably such as tomaintain the concentration of polymer in the furfural in the system atbelow 1% by Weight.

In accordance with the present invention lime is admixed with thefurfural in the extractive distillation system, for example, bysuspending it therein in any suitable Way. It is preferred to limit theamount of lime to that required to keep the acidity at the desiredlevel. The amount of lime required is so small that in practice nodifculty due to sedimentation of the lime or its reaction products isencountered. The lime or a portion thereof may dissolve in the furfural;the presence of dissolved water in the furfural may enhance any suchtendency.

A very convenient method of contacting the furfural with the lime is toemploy an excess of a dispersion of lime in water, admix it veryintimately with the furfural in any suitable Way, for example, Aby meansof a centrifugal mixing pump, run the resulting churned-up mixture oremulsion into a settling tank where it is allowed to separate into twolayers, an upper layer of the water and lime which have not dissolved inthe furfural and a lower layer of furfural which is saturated with waterand which may contain dissolved lime. These layers are separatelywithdrawn, the Water layer being recycled to the mixing zone to save itswater, lime and dissolved furfural content and the furfural layer beingfed to the absorber. Under conditions where corrosiveness of thefurfural in the system can be kept down to a satisfactory level such atreatment as just outlined may be applied to a sidestream of the recyclefurfural, the resulting non-corrosive furfural being intimately admixedwith the balance of the recycle furfural and the furfural from there-run unit and allowed to stand as in a surge or storage tank for aperiod of time before being re-introduced to the absorber.

aasn

Alternatively an excess of powdered lime may be intimately admixed withthe furfurai and after a suitable period of time separated therefrom byany suitable means such as filtration, centrifuging o r sedimentation. v

It is preferred to introduce enoughlime into the furfural to reduce theacid content of the furfural in the extractive distillation system below0.06 per cent by weight, measured as acetic acid. and still morepreferably below 0.04 per cent by weight.' I have found that below 0.04per cent of acidity no appreciable corrosion occurs; between 0.04 and0.06 per cent acidity the concentration of the corrosive substancewhatever it is may or may not be such that serious corrowsion occurs,apparently depending upon factors not understood at present; while, atan acidity above 0.06 per cent corrosion always occurs and is usually sohigh that the equipment, especially the fractionating columns (i. e. theabsorbers) and the strippers, would soon be eaten away below the point'Iof safe operation if the plant continued to operate at such levels ofacidity. Furthermore, the loss of furfural by polymerization at levelsof acidity above 0.06 per cent becomes excessive so that furfuralconsumption exceeds the available supply which is exceedingly dangerousto the synthetic rubber program. 'Ihe relationship between level ofacidity and polymerization of furfural is at present not understood andI do not wish to be limited to any theory with regard thereto.

It is preferred to introduce the lime to the furfural at a point in thesystem such that the furfural is at a temperature of not over 130 F. atthe point where the admixture takes place. Usually the coldest point inthe furfural system is just after the cooler or coolers which areemployed to cool the furfural leaving the stripper or strippers and soit is preferred to add the lime at this point. Ihe reason for adding thelime at a point of low temperature is that the possibility of additionalpolymerization of fu'rfural upon the introduction of lime to hotfurfural is thereby eliminated.

The purpose of lime addition is to reduce the acid content of thefurfural, reduce the formation of polymer and the corrosiveness of thefurfural. The manner in which these several factors are inter-related isnot understood but it has been deiinitely established that the additionof lime does prevent the furfural from being corrosive and does reducethe polymer formation, whether one causes the other or not. The presentinvention may be employed from the start of operations to keep thefurfural in the system from ever becoming corrosive or may be applied tosystems in which the furfural has become corrosive to eliminate thecorrosivity and keep the system thenceforth non-corrosive. Thus theprocess of the present invention prevents noncorrosive furfural frombecoming corrosive and also prevents corrosive flirfural from exertingits deleterious action. The addition of lime in accordance with thepresent invention involves more than simple neutralization of corrosive`acidity since it inhibits polymer formation as well. Moreover, it hasbeen found by experimental work that neither the acid content nor the pHof furfural is necessarily an index of its corrosiveness. It will beobvious that acid content and pH are not the same since it is common tohave a high acid content associated with a pH ranging from to 'I or alow acid content and a pH ranging from 0 to '1.

lil

.As indicated above, the acid content of the furfural is not necessarilyan absolute index of its corrosiveness. It is believed that theconcentration of corrosive material in the furfural increases somewhatin proportion to the acidity.

.At levels of acidity greater than 0.04 per cent it is evident that somefactor other than acidity comes into play as a cause of corrosion sincebetween levels of 0.04 and 0.06 per cent acidity serious corrosion mayor may not occur and at levels of acidity above 0.06 per cent thecorrosion which occurs is not always serious although it usually is. Inany event, whatever the causes of corrosion and regardless of theory ithas been proven absolutely that by addition of lime to the furfural inthe system in an amount such as to reduce the acidity to, or maintainthe acidity at, levels below 0.04 per cent serious corrosion isprevented and polymerization is inhibited; while reduction of acidityto, or maintenance at, levels lbetween 0.04 and 0.06 per cent by theaddition of lime in most cases prevents serious corrosion and reducedpolymerization of furfuralto satisfactory levels.

In accordance with the present invention the acid content may be reducednearly to or to the neutral point or the furfural may be made alkalinealthough the latter practice is not ordinarily followed because thetendency of furfural to polymerize is enhanced under alkalineconditions. It is preferred to add only enough lime to reduce the acidcontent below 0.06 per cent and preferably below 0.04 per cent whichpractically eliminates corrosion of carbon steel by the furfural,however, the'nearer the neutral point or the point of zero acidity theless polymers of furfural will be formed. When the acid content offurfural in the main system is below 0.04 per cent the corrosion ofcarbon steel equipment is negligible and the rate of formation ofpolymer is substantially lower than when the acid content is above 0.06per cent.

The products formed from the reaction of lime are removed from thesystem in the re-run unit with the polymers, so no problem due toaccumulation of lime or its reaction products is encountered. Inordinary operation the re-running is carried out at such a rate that thefurfural in the entire system is re-run in a short time say every two tothree days. If enough lime is added to cause a sediment to form in themain system it may be necessary to provide a trap or filter to removethe sediment from the furfural. My invention may Ibe applied by addinglime to a small side stream of furfural and any sediment that might beformed may be removed by filtering or some other method before thefurfural is returned to the main stream. However, it is often desirableto add only enough lime to reduce the acid content of the furfural tothe extent indicated but not enough to result in difllculties due tolime or the reaction products of lime settling in the system.

In accordance with my invention the fresh furfural which is added asmakeup to the system may be treated with lime prior to its introductioninto the extractive distillation system.

In accordance with the present invention lime is added in the mainsystem, i. e. in the extractive distillation system proper. asdistinguished from the re-run unit. 'I'he addition of lime to the re-rununit is disclosed and claimed in my copending application Ser. No.607,212, filed July 26, 1945.

It is often desirable to apply the present invention to extractivedistillation systems wherein furfural has become excessively corrosive.In such case it may be desirable to add enough furfural -in the mainsystem to reduce the/acidity 0.02 per cent each day until it is below0.04 per cent and to then reduce the amount of lime added to a ligurewhich will hold the acidity at this level.

Referring to the accompanying drawing, I have illustrateddiagrammatically the application of my invention -to a plant'in whichbutadiene is made by the two-stage catalytic dehydrogenation of normalbutane. The mixture of normal butane and butene-Z recovered from step Iof the process enters absorber 2 vla line I. while the C4 hydrocarbonsderived from step III and containing butene2 and butadiene enterabsorber l via line 3. Absorbers 2 and 4 carry out the extractivadistillation of the feed with furfural injected into the top via lines 5and 8, respectively. Absorbers 2 and I are provided with conventionalreboiling means and means for condensing and returning a portion of theoverhead as reflux; the latter being indicated diagrammatically by acooling coil in the top of each absorber. 'I'he undissolved normalbutane leaves absorber 2 via line 1, while the furfural rich in butene-2is withdrawn as bottom product via line 8 and fed to stripper 9 wherethe dissolved butene-2 is driven oil'. Stripper 9 is likewise providedwith conventional reboiler and means for condensing and returning aportion of the overhead as reilux.

The undissolved C4 hydrocarbons leave absorber 4 via line I0 while thefurfural rich in butene-2 and butadiene passes via line Il to stripperl2 f which performs the same function as stripper 9, the butene-Z andbutadiene mixture leaving via line I3. The stripped furfural streamsIleaving stripper 9 via line Il and stripper I2 via line I5, are mergedand fed via line I0 to cooler l1. It will be understood that a coolermay be provided for each stripped furi'ural stream if desired. The

cooled furfural flowing in line I8 is passed via lines I9 and 20 tofurfural surge or storage 2| whence it is recycled via line 22 and linesI and to the absorbers. A side stream of furfural is passed via line 23to lthe re-run unit 24 where it is .continuously puried, in the mannernow well known in the art, to remove the polymer therefrom. The re-runfurfural is passed via line 25 into the main recycle stream in line I9.

In accordance with the present invention lime is continuously fed in theproper amounts indicated above into .the main recycle stream owing inline I9. This is accomplished by means of line 26 which, it will beunderstood, is purely diagrammatic. Usually both lime and water areinjected into the stream in line I9. As explained above it is oftenconvenient to inject the makeup water at this point. A point ofinjection of lime close to cooler I1 is preferred because thetemperature is at substantially the lowest level at this point andbecause this gives arelatively long line between the point of injectionof lime and furfural Surge 2l. lime with furfural may take place in `thelong pipe indicated by lines I9 and 20.

It will be understood that the showing in the drawing is purelydiagrammatic and that the lime and water may be added separately or inany desired manner and that provision may be made for the removal of anyexcess lime or sediment. It may or may not be necessary to provide forthe removal of the reaction products of lime and .the impurities in thefurfural, depending upon the amount of lime added and -the rate at whichfur- Intimate mixing oi' the 8 Y tfural is passed to the re-run unit 2lfor purincaon. Y

Example In the manufacture of butadiene by two-stage dehydrogenation ofnormal butane, a recovery system, as shown in the drawing, was employed.Furfural was fed continuously into the absorbers 2 and 4 at the rate of260,000 gal/hr. entering both absorbers. The furfural contained 8% ofwater by weight based on the weight of furfural and water. 'I'hestripped furfural from both strippers was cooled to 1Z0-130 F. and themaior part of the cooled product was passed to the surge tank 2|. A sidestream of 3500 gal/hr. was withdrawn via line 29 and fed to re-run unit2|. The total volume of furfural in the entire system was 225,000gallons. The acidity of the furfural in the main system was 0.10 weightper cent measured as acetic acid. This was exceedingly corrosive to theequipment and polymer formation was excessive. The acidity had beenapproximately 0.10% for ten days, during which time' the polymer contentincreased from 0.4 to 1.5 per cent by weight. Under these conditions,namely at 0.10% acidity, 1.5% polymer concentration and 3500 gaL/hr.v offurfural re-run, equilibrium had been established. However, operationswere unsatisfactory and it was considered imperative to reduce corrosionand polymerization because otherwise the plant would be corroded away ina short time and it would be impossible to obtain suilicient furfural tocontinue operations.

Under the foregoing conditions, lime was added to the furfural in themain system at a rate of 11 lbs./hr. The lime was added in the form of aslurry in the makeup water which was employed to keep the waterconcentration at approximately 6%. The slurry was admixed with thefurfural recycle stream immediately after the cooler, as shown inthedrawing. 'I'he acidity immediately began to decrease at the rate ofapproximately 0.015% per day'. When the acidity reached 0.04% the rateof addition of lime was decreased to 6 iba/hr. and the acidity variedfrom 0.03y to 0.038% from -then on for an indenite period. 'I'he polymercontent had decreased to 0.3% and remained at about this figure.Simultaneously with the attainment of 0.04% acidity, the rate ofre-running of furi'ural was reduced to 2000 gal./hr.which suftlced tokeep the polymer concentration from increasing above 0.3

As a result o1.' the use of lime in the manner described in theforegoing example the loss of furfural in the plant was reduced from4000 to 1500 gaL/day.

The amount of lime necessary to be added t0 the furfural in accordancewith the present invention in'order to keep the furfural from becomingcorrosive and the polymer content thereof from increasing is extremelysmall. Under ordinary conditions of operation, it ranges from 0.000005to 0.000006 lbs/gal. of furfural fed into the absorber per hour. Thus,the .total pounds of lime in the system at any one time is very small.usually less lthan 10001bs. for a system containing 225,000 gallonsfurfural as in the example.

From the foregoing, many advantages of the present invention will beapparent to those skilled in the art. The principal advantage is thatcorrosion of -the equipment in the main system is prevented. This allowsthe use of equipment of carbon steel which is much less expensive thancorrosion-resistant metals. An-

9 other major advantage of the present invention is that it inhibitspolymer formation in the main system which of-itself gives rise to manyadvantages well known to thoseskilled in the art. The

invention accomplishes simultaneous reduction in the level of acidconcentration in iurfural in the extractive distillation system andreduction in the rate of polymerization of iurfural. The invention issimple and can be carried out without requiring expensive additionalequipment or introducing complications into the existing extractivedistillation system and re-run unit. By using lime, there is no dangerof high concentration of alkaline material at any point which would beharmful because it would cause an unduly high rate of polymerization of4furfural; lime is unable to give a highly alkaline condition by reasonof its nature and limited solubility in iuriural. The invention isextremely economical to apply because of the small amount of lime andbecause lime is cheap. The invention greatly reduces the amount offurfural which has to be re-run in order to remove polymer.` Thisreduction in amount of furfural re-run gives rise to many otheradvantages, the principal of lwhich is the markedly reduced loss oi'furiural and the lowered cost of re-running the smaller amount o!furiural. It will be understood by those skilled in the art that loss offurfural inthe re-run system A is high because the re-running itselfcauses additional polymerization andv because itx is necessary to leavea considerable amount oi'- iurfural in the polymer in order to get itout of the re-run unit.

The invention is further advantageous in that it accomplishes preventionof formation of impurities at the very point where such impurities aremost deleterious, namely in the extractive distillation system whichembodies very expensive absorber and stripper units. A very markedadvantage of the present invention is that it causes no foaming of thefurfural in the extractive distillation system. Foaming of furiural inthe absorbers and strippers is most objectionable. The behavior of limeintroduced in accordance with the present invention is to be comparedwith that of soda ash, sodium bicarbonate or sodium hydroxide.Undoubtedly such sodium compounds would, if added to the furiural in themain system, substantially decrease corrosion and formation oi'polymers; however, such alkaline-reacting alkali metal compounds causethe iurfural to roam which cannot be tolerated in processes ofabsorption and stripping. 'Even traces of sodium hydroxide, carbonate orbicarbonate cause foaming of furiural so that it would be completelyimpossible to use such materials in place of lime in accordance witl'thepresent invention.

Another advantage of the present invention is that the amount o1 limerequired is so small that no problem due to sediment in the main systemis presented. The lime or its reaction products 10 is completely removedfrom the furfural in the re-running operation. Many other advantages ofthe process of the present invention will be apparent to those skilledin the art.

I claim:

1. In the process of separating unsaturated aliphatic hydrocarbons froma mixed hydrocarbon stream containing such unsaturates by extractivedistillation wherein said mixed hydrocarbon stream is passed upwardthrough an absorber tower in countercurrent to' a descending stream ofsolvent composed of furfural containing dissolved water, a bottomsproduct consisting of furfurai rich in unsaturated hydrocarbon iswithdrawn from said absorber tower, said bottorn product is introducedinto a stripper column and there stripped of its unsaturated hydrocarboncontent, a bottoms product consisting of stripped furfural is withdrawnfrom said stripper column and split into two streams, one of suchresulting streams is passed to a re`run unit and there distilled to givea puried material consisting essentially of furfural and water dissolvedtherein, the purified furfural is combined with the other streamerstripped furfural and the,

resulting combined furiural stream is recycled to said absorber tower assolvent for the extractive distillation step; that improvement whichiurfural is fed into said absorber tower; and

controlling the amount of lime so introducedso that the acidity of thefuriural in the extractive distillation system is maintained between0.02

per cent and 0.06 per cent by weight, calculated as acetic acid. v

2. The process ofk claim l wherein the lime is introduced into thefurfurai in the form of an aqueous dispersion. v

3. The process oi claim 1 wherein the introduction of said lime isaccomplished by adm ixing same with said branch of the'stripped furfuralstream which bypasses the re-run unit while said furfural is at atemperature of lnot over 130 F.

4. The process of claim 1 wherein said hydrocarbon stream is analiphatic C4 stream and wherein saidunsaturated hydrocarbon is an all'-phatic C4 hydrocarbon.

5. The process of claim 1 wherein said unsaturated hydrocarbon comprisesbutene-2.

6. The process of claim 1 wherein said unsaturated hydrocarboncomprisese butadiene.

Virton. scAR'rH.

